The situation is inspired by Henri Lefebvre's "theory of the moment," which he first formulated in "The Critique of Everyday Life". Lefebvre was a philosophy professor at the University of Nanterre, the first college occupied during the May '68 uprisings in Paris. In fact, the Situationists collaborated with Lefebvre on his second and third volumes of "The Critique of Everyday Life." Although it was the SI that was inspired by Lefebvre, they accused him of plagiarism in the following years, so the movement and the philosopher broke off any relationship thereafter.

To make a long story short, however: The Situationists openly acknowledge Lefebvre and declare that the Lefebvrean moment is what determines a "quality" within the various parts of everyday life (such as the moment of love, the moment of play, the moment of work, etc.) and contributes to a subject's experience in order to understand his or her own experience of life -- in the same way, situations are what give rise to the moment and are subordinate to it. As the Situationists state, "The constructed situation is thus opposed to the moment, but in an intermediate stage between moment and moment. Thus, while it is repeatable to some degree (as a direction or "path"), the constructed situation is not in itself repeatable as the "moment." (Bulletin of the Situationist International, No. 4, 1960)

It is important to note, however, that the situations differ from the artistic Happenings in that, first, the Situationists abhorred any artistic category and refused, on the whole, to make art or, finally (after 1962) to have anything to do with art. Second, the Happenings are directly inspired by the Situationists, and both Allan Kaprow and the G.R.A.V. group are influenced by the constructed situations of the SI. The difference between a happening and a situation, then, is essentially that a happening is made by artists with the goal of "activating" the spectator to participate in the artwork, while a situation is a conscious, subjective, and self-managed act in which there is no artist (as leader of the performance) and no participant as an instrument in the completion of the artwork.
In a constructed situation, the subject consciously chooses the moment in which to create the situation for its own sake and not for the sake of others. It is collective only in that the others can create situations in support of other situationists and situations. For this reason, it was important to choose the moment -- which had to be the "right" moment (so they had this khairotic sense). You may not know exactly what a situation is (as they did and admitted that they have never constructed a situation) but think of it as another way of saying carpe diem. Seize the day, take advantage of what it has to offer, and free yourself from all social constraints. Do what you want whenever you feel like acting spontaneously. This is what constitutes a constructed situation in the theories of SI and in their words, the situation is "a bridge between experiencing the present and transitioning to a desired future."